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Even as the Mitsubishi Ki-21 was entering service, the Nakajima com-pany was issued with the Ki-49 specification early in 1938 calling for an aircraft to replace the Ki-21, capable of reaching 500 km/h (311 mph) with a range of 3000 km (1860 miles) and a bombload of 1000 kg (2200 lb). Defensive armament was to include a free 20-mm (0.79-in) cannon on a dorsal mounting and a tail gun turret, the latter for the first time on a Japanese landplane. It was also laid down that adequate protection for the crew members and the fuel load was to be provided.

Nakajima set to work in mid-1938 with a top-level design team led by Yasumi Koyama. A mid-wing layout was selected, with a considerable dihedral on the low aspect ratio wing. The wide-chord centre section accommodated six fuel tanks, three on each side of the fuselage. Good takeoff and climb were guaranteed by large Fowler -type flaps. A 20-mm (0.79-in) Ho 1 cannon was mounted in the dorsal position, while single 7.7-mm (0.303-in) Type 89 guns were fitted in the nose, ventral and port and starboard beam positions and in the powered tail turret.The first prototype flew in August 1939. Its two 708kW / 1080-hp Nakajima Ha-5 Kai 14-cylinder radial engines were replaced by Nakajima Ha-41s of 932kW / 1250 hp each on the remaining two prototypes and seven evaluation aircraft delivered late in 1939. Constant-speed three-bladed propellers were introduced in place of the original Hamilton Standard two-pitch propellers.

The Ki-49 was accepted for ser-vice as the Army Type 100 Heavy Bomber in March the following year and made its operational debut in February 1942. Later code-named 'Helen' when the initial production Ki-49-I (Army Type 100 Heavy Bomber Model 1) started operations with the 61st Hikosentai in China (powered with Ha-41 radial engines).

The army, fearing heavy losses in unes-corted daylight raids, tested three modified Ki-49s, intended as escort fighters. Designated Ki-58, they were fitted with improved armament and crew protection but with the bomb bays deleted. Progress was disappoint-ing and then the Ki-43 fighter, possessing sufficient range for escort work, became available in quantity and the Ki-58 pro-gramme was abandoned. Two Ki-49 variants, designated Ki-80, were intended as prototype formation-leader aircraft, but this idea was also abandoned, and these aircraft were then used to test the powerful new 2420-hp Naka-jima Ha-117 radial engines.

Known in Japan as Donryu (Storm Dragon), Ki-49-Is went first to equip the 61st Sentai operating in China. During the Pacific war Ki-49s, coded Helen by the Allies, flew frequent raids over New Britain and New Guinea, also making a large proportion of the attacks on Port Darwin and Australia's Northern Territory.Two preproduction Ki-49-IIs appeared in September 1942, powered by twin 1450-hp Nakajima Ha-109 radials. In the spring of 1942 the steps were taken to increase performance, protection and defensive firepower. The Nakajima Ki-49-IIa (Army Type 100 Heavy Bomber Model 2A), the first of the new series, was powered by two Nakajima Ha-109 radial engines with increased ratings. The aircraft was fast, well protected by 5mm armour plating and rubberized fuel cells, and heavily armed. The Ki-49-IIa was delivered to the Sentais from August 1942 onwards and the later Ki-49-IIb variant had 12.7-mm (0.5-in) Ho 103 machine-guns replacing the light machine-guns for-merly installed in the nose, ventral and tail positions. The Ki-49-II operated mainly over New Guinea and in China, as well as being assigned to units deployed in Manchuria against an anticipated Soviet attack.

Both the Ki-49-IIa and Ki-49-IIb were in action with the 7th and 61st Sentais based in China in the summer of 1942, and made many attacks on Chungking and air bases of General C. L. Chennault's China Air Task Force. 'Helen' bombers equipped the 12th Sentai, based at Medan and Sabang (Sumatra) under the 3rd Air Army, and these attacked objectives in Burma and eastern India, joining Ki-21s on some occasions in raids on Calcutta. In 1943 the Ki-49s of the 7th and 61st Sentais, now operating from Timor, attacked Darwin to face strong reaction by the Supermarine Spitfire Mk.Vs of No.1 Fighter Wing. Units equipped with Ki-49s suffered most in New Guinea, where 4th Air Army came under constant attack on its airfields at Wewak, But and Dagua after August 1943, when the RAAF and the US 5th Air Force went onto the offensive.

Ki-49s also flew many missions during the desperate Japanese defence of the Philippines, and surviving aircraft, with a reduced crew of two, made suicide attacks on Allied ships. Some Ki-49-Is were fitted with detection equipment and used as antisubmarine aircraft, while others were pressed into service as troop transports.

Six examples of the final version, Ki-49-III, were built, powered by Nakajima Ha-117 radials, each intended to provide 1805kW / 2420 hp. The engines were never really more than experimental, however, and technical problems remained unsolved. Plans for mass production of the Ki-49-III came to nothing.

In all, 819 Ki-49s of all variants were built, 769 by Nakajima at its Ota factory in Gumma Prefecture and the remaining 50 by the Tachikawa company.